The present invention generally relates to an optical pickup head, and more particularly to an optical pickup head which utilizes several laser beams of different wavelengths for reading or writing data on different kinds of optical disc.
An optical pickup records and reproduces information such as video or audio data onto/from recording media, e.g., laser discs. A disc has a structure that an information-bearing surface is formed on a substrate. Regular compact disc drivers are designed subject to the specifications of the laser discs. Currently, a DVD (digital versatile disc) driver is designed to read/write data on different kinds of optical disc, such as CD (compact disc) or DVD. The reading or writing is made by an optical pickup head which can provide two focusing points on an information-bearing surface of the disc which has a specific thickness. The thickness of the disc is defined as a distance from the surface of the disc to the information-bearing surface. When reading, the laser beam reflected from the disc and bearing the data signal is transformed by a photo detector into recognizable electrical signal. A similar process is generated for data writing.
The following is a table listing the specifications of the disc and the laser light for a CD and a DVD:
NA THICKNESS TRACK WAVELENGTH (numerical (mm) PITCH (.mu.m) (nm) aperture) CD 1.2 1.6 780 0.45 DVD 0.6 * 2 0.74 635-650 0.6
Because the thickness of a CD is different from that of a DVD, for the requirement of compatibility, a DVD driver must be designed to read discs of different thickness. There are several methods which achieve this requirement. These methods include:
1) using two objective lenses with different focuses, and selecting one of them corresponding to the CD or DVD by a driving mechanism such as a rotary mechanism. But the two objective lenses greatly increase the weight of the pickup head and the price of the product; PA1 2) providing a diffraction member for the objective lens and performing two focuses; PA1 3) using a HOE (holographic optical element) to provide two focusing points. But the efficiency of usage of laser light is decreased, and the HOE is difficult and expensive to be manufactured; PA1 4) using an LCD (liquid crystal display) shutter as an NA controller which produces two numerical apertures from a single wavelength laser. However, it is difficult to be installed since the polarization of the laser light source must be properly arranged relative to the LCD and polarizer. Moreover, the LCD needs continuous power supply to maintain shutter effect; PA1 5) providing an objective lens having a near axial region which includes a center of the light path and a far axial region located radially outward from the intermediate region, using electric signals corresponding to the near axial region converted in only the inner photodetector when the light is reflected from a CD; and using electric signals corresponding to both near and far axial regions converted in both the inner and outer photodetector when the light is reflected from a DVD. This is disclosed in U.S. Pat. No. 5,665,957. It has the disadvantages of higher manufacturing cost, lower acceptable tolerance or higher precision demand of installation, and decreasing the laser efficiency about 15%; PA1 6) as disclosed in U.S. Pat. No. 5,777,970, providing a first optical system which leads the laser beam output from a first laser diode and a second optical system which leads the laser beam output from a second laser diode, providing an optical element which has refractivity to either the first optical system or the second optical system, and driving either the first laser diode or the second laser diode according to the type of the loaded optical disc. The drawbacks are that it is expensive and complicated in structure; and PA1 7) adjusting an electronic aperture ring to change the numerical aperture, and change the distance between the disc and the object lens subject to type of the disc used. Similar designs are disclosed in U.S. Pat. No. 5,659,533 and U.S. Pat. No. 5,281,797.